The present invention relates to the field of oil and gas production equipment, and more particularly to the field of wellhead equipment. More particularly still, the present invention relates to the field of casing hangers and seal assemblies for locating and sealing casing in a wellhead.
Local, state, federal, and foreign laws and regulations commonly require that the sidewalls of oil and gas wells be sealed to prevent the migration of oil and gas from the reservoir area into aquifers and adjacent soil. In order to isolate the wellbore from the surrounding soil and prevent collapse from formation pressures, casing is inserted into the well and cemented to the walls of the wellbore. The casing is typically comprised of joints of hardened steel tube, a string of which in a wellbore may weigh a million pounds or more. The casing strings must be supported in the wellbore to prevent them from collapsing into the well. Further, the upper end of the casing must be sealed to prevent well fluids from escaping past the end of the casing and into the atmosphere or soil. To perform these functions, wellhead assemblies have been devised which can support and seal several casing strings in annular alignment in the wellbore.
In a typical well, a tool known as a hole opener opens a shallow bore to start the well and a string of conductor pipe is then driven or drilled into the earth. The hole for the surface casing is drilled through the conductor pipe. The surface casing is run into the well and cemented in place, and a landing base may be installed at the upper end of the conductor pipe. A head is installed on top of the surface casing, on the landing base, if present. Successively smaller diameter and deeper holes are then drilled for one or more successively smaller diameter but longer intermediate casing strings, down to the production casing. These casings are in turn run into the wellbore, cemented, and suspended in the wellhead. The next smaller size casing from the surface casing is typically suspended from hanger equipment in a casing head, and successively smaller casing strings are typically suspended from hanger equipment in separate spools or heads installed on top of the previous casing heads. Then, after all casings are in place, a similar procedure is followed for the tubing head and tubing string or strings. Each time a new one of the separate heads is installed the blowout preventer equipment, which is usually in place during drilling operations, is removed. During the time the blowout preventer equipment is removed, the hole is exposed and unprotected from blowouts until the new head is installed and the blowout preventer stack reinstalled or replaced. Therefore, it is important that installation of the several heads comprising the wellhead assembly be effected as rapidly as possible.
Each head of the wellhead assembly comprises an annular member having a load shoulder or taper therein which engages a bowl and/or slip system to engage the casing string and support it in the wellbore. A seal assembly is also employed to seal the annular space between the casing and the head. One of three head profiles is typically employed to hang the casing string in the wellbore: a taper; a multiple shoulder; or a single shoulder. In each design slips engage the upper end of the casing string and are actuated radially inwardly when pulled down an angled surface in the head or the bowl by the casing. When equilibrium is reached the tension on the casing will be transferred via the slips and other hanger components into the head without adverse effects on the casing.
In practice, the hanger assembly is wrapped around the casing which protrudes out of the head or the blowout preventer stack. The assemblies are then lowered or dropped through the blowout preventer (if present) into the head. The casing is pulled upward in the well, which stretches the casing. The force pulling on the casing is then incrementally reduced, tending to reduce this stretch. At this point, the slips, which are in contact with the casing, are pulled into the bowl or head. The movement into the taper of the bowl or head produces a radially inward actuation of the slips, and a circumferential gripping of the casing by the slips and the slips by the bowl or head. The slips, having been radially actuated by the downward vertical movement, resist further movement of the casing and hold it in stretched tensional equilibrium in the wellbore. The action of the slip and bowl system causes a seal assembly located adjacent thereto to compress, forcing the seal to radially engage the annularly opposed casing and head surfaces.
The tapered head casing hanger with top seal employs a head having a tapered profile which interferingly engages a set of slips which in turn support the casing string. A seal ring assembly is located over the slips to form a seal between the outside diameter of the casing string and the inside diameter of the head above the tapered area when the seal is actuated. The hangers include slips (inner component) and a bowl (outer component), each comprised of a number of sections placed end-to-end to form an annular structure. The slip assembly is generally cylindrical, and includes a minor diameter lower portion, a major diameter upper portion, and a frustoconical-shaped slip shoulder on its outer diametral surface connecting the minor diameter and major diameter portions. The inner diametral surface of the slip assembly includes a plurality of radially inwardly extending teeth which engage the casing. The bowl, having a frustoconical-shaped shoulder on its inner diametral surface, a frustoconical-shaped lower outer surface, and a flat upper face, is disposed between the slip assembly and the head. The bowl terminates on its upper end in a flat annular ledge, upon which a seal assembly is disposed. The seal assembly is a sandwich seal, having opposed upper and lower plate sections and a seal element located therebetween. A plurality of bolts or cap screws are anchored in the upper plate and pass freely through the lower plate and seal element, and are threaded into the slips. As the casing and slips are moved into the head, the slips engage the bowl, forcing the bowl further into engagement with the head. The bowl will ultimately seat against the tapered portion of the head and the slips will move downwardly into the bowl until the slip shoulder of the slip assembly engages the frustoconical-shaped shoulder on the inner surface of the bowl. As this occurs, the lower plate of the seal assembly will bear on the flat upper face of the bowl as the upper plate is pulled down via the bolts or cap screws anchored in the upper plate and attached to the slips. As a result, the seal element compresses vertically and expands radially into engagement with the casing and head to seal the annular area therebetween. The tapered head top seal casing hanger has a significant disadvantage. When pressuring the annulus from the bottom (slip side), the seal can pull the hanger upwardly out of the tapered portion of the head, causing the slips to disengage from the casing and permitting the casing to slip and buckle in the well.
A multiple shoulder casing hanger with automatic seal employs a head having an upper radial shoulder upon which a sandwich seal is seated, and a slip and bowl system therebelow for holding the casing string and energizing the seal. The head includes a lower minor diameter portion and an upper major includes a lower minor diameter portion and an upper major diameter portion interconnected by an upwardly facing frustoconical support shoulder. A bowl, having a major diameter, a minor diameter, and a downwardly facing shoulder for mating with the support shoulder of the head, is disposed below the seal. The seal may be bolted to the bowl in a manner like that referred to above in connection with the tapered head casing hanger with top seal. The inner diameter of the bowl comprises a continuous, upwardly facing frustoconical surface. A set of slips, in the form of a circular wedge, has a gripping face in contact with the casing string and a second gripping face in engagement with the continuous frustoconical surface of the bowl. The bowl is sized to permit the bowl to actuate or travel downwardly as the casing string is engaged by the slips, thus energizing the seal. The support shoulder in the head forms a stop to limit the downward travel of the bowl, thus limiting the linear actuation of the seal. This allows part of the weight of the casing string to be carried by the seal assembly and its radial support shoulder on the head, and part on the frustoconical support shoulder between the bowl and the head.
Because multiple shoulders are employed in the head, the multiple shouldered hanger requires high tolerance machining of the head to properly locate the bowl stop shoulder and seal support shoulder relative to one another. Loss of shoulder surface area due to tolerancing can cause problems in holding applied loads, and these problems become critical for higher hanging loads. In addition, no means exists to seal below the slips and isolate pressure from the slip area. Moreover, when higher pressure rated heads are used, profile changes in the heads require profile changes on the outside of the hangers forcing higher inventory requirements.
A single shoulder casing with automatic seal employs a head having a radial shoulder which accepts the load of the seal and the pipe weight. The assembly includes a set of wedge-shaped slips which engage a bowl disposed in a head, and a seal disposed in annular sections on either radial side of a pedestal mounted below the bowl and retained on an annular ledge in the head. As the slip is forced downwardly in response to casing loading, the base of the bowl, which straddles the pedestal, squeezes the seal. To prevent over-energization of the seal, the bowl includes a slot which is shallower than the height of the pedestal and into which the pedestal projects. Thus, the bowl ultimately rests on the pedestal at complete seal actuation. As load is applied to the seal from the weight of the casing string, the seal elements are squeezed and expand radially to fill the gap between the head and the pedestal, and the pedestal and the casing. Large tolerance ranges for the outer diametral surface of the casing can create from very high to very low sealing loads, given that the inner seal does not communicate with the outer seal. This can create problems ranging from overstressing the casing to not producing a good seal and often leads to uneven compression loads between inner and outer seal.
The single shoulder hanger may have a seal without any support in the seal element. This would leave the seal holding the hanging weight. Overstressing of the seal and the pipe could lead to pipe collapse or excessive seal extrusion.
Thus, the prior art casing hangers have several deficiencies. For the tapered head style, perhaps the greatest technical shortcoming is the possibility of dropping pipe when pressured from below. This can cause severe damage to the well, and to the pipe, the rig, or other hardware, and can be extremely hazardous to personnel. Obtaining and maintaining adequate shoulder area is perhaps the greatest technical problem associated with the multiple shoulder head style. With higher hanging loads and pressures special heads and hangers are required to keep deformation of head and hanger to acceptable levels. Many consider the major technical problem with the present single shoulder style to be achieving and maintaining proper seal loading. Without a stop, the seal can be overpressured when required to support hanging weight. When the prior art annular pedestal is employed, two seals are required. The inner seal must cover a very wide range due to the amount of tolerance on the casing. As a result, there may be a very high stress in one seal, and a very low stress in the other seal, with no adequate means of equalizing them.
An economic burden common to all of the foregoing designs is the need for high part count. This produces a need for higher inventory count, and as a result, higher costs.